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Generators

Generator Sizing Calculator

Build a load table, mark which loads are motors and how they start, and get a minimum generator rating in kVA. The result also shows whether running load or motor starting sets the size, and suggests the next size from a common list of genset ratings.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Load table

  • ×
  • ×

Default starting multiples: DOL 6×, soft starter 3×, VFD 1.5× running kVA. Edit them to match motor data.

Design factors

1 = everything runs at once

%

Typical 20–25 % for growth

%

From the maker's derating table

0.8 is the usual rating basis

× kVA

1.0 = conservative; use maker's curve

Result

Generator sizing — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Minimum rating needed
124.3kVA
Next common size
125kVA

Governed by motor starting (Water pump). At PF 0.8 the minimum rating is 99.41 kW.

Connected running load
32.5 kW / 35.76 kVA
Diversified running load
32.5 kW / 35.76 kVA
Running requirement incl. margin & derate
48.75kVA
Peak kVA during largest start
124.3kVA
Starting requirement
124.3kVA

Working, with your numbers

  1. 1. Connected running load
    S = √((ΣP)² + (ΣQ)²)
    S = √(32.5² + 14.9212²)
    = 35.762 kVA (32.5 kW)
  2. 2. Running kVA the set must cover
    max(S_d, P_d / PF_gen)
    max(35.7616, 32.5 / 0.8)
    = 40.625 kVA
  3. 3. With margin and derating
    S_run = S × (1 + m) / (1 − d)
    S_run = 40.625 × (1 + 0.2) / (1 − 0)
    = 48.75 kVA
  4. 4. Starting kVA of largest step (Water pump)
    S_start = multiple × S_motor
    S_start = 6 × 17.6471 kVA
    = 105.88 kVA
  5. 5. Peak during that start
    S_peak = S_other running + S_start
    S_peak = 18.3818 + 105.882
    = 124.26 kVA
  6. 6. Rating needed for starting
    S_st,req = S_peak / capability / (1 − d)
    S = 124.264 / 1 / (1 − 0)
    = 124.26 kVA
  7. 7. Minimum rating (larger of the two)
    S_req = max(S_run, S_st,req)
    max(48.75, 124.264)
    = 124.26 kVA → 125 kVA common size

Assumptions

  • Running real and reactive power are summed separately, then combined: S = √(P² + Q²).
  • Motors start one at a time; the motor producing the highest peak starts last with all other diversified load running.
  • Starting kVA = multiple × motor running kVA, added arithmetically to the running kVA of the other loads (conservative because starting PF is low).
  • Starting capability 1 × rated kVA (1.0 gives no credit for the set's short-term starting ability).
  • Margin 20 % applies to running load; derate 0 % reduces available capacity for both running and starting.
Load 1: Lighting & sockets: 1 × 10 kW, PF 1
Load 2: Water pump: 1 × 15 kW, PF 0.85, motor Direct on line (DOL) 6×
Load 3: Ventilation fan: 1 × 7.5 kW, PF 0.8, motor Soft starter 3×
Diversity: 1
Margin: 20 %
Derate: 0 %
Generator PF: 0.8
Starting capability: 1 × rated

Check before you rely on this

  • Final selection must use the manufacturer's sizing software or motor-starting curves (kVA at an acceptable voltage dip) and the set's prime/standby rating definitions (ISO 8528-1).
  • Motor starting governs the size. A soft starter, VFD or staged starting sequence may allow a smaller set.
  • No site derating entered. Generator output falls at high altitude and high ambient temperature; take the derate % from the manufacturer's data.
  • Non-linear loads (UPS, rectifiers, VFDs) may need an oversized alternator; this is not modelled.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

How to use this calculator

  1. Add one row per load or group of identical loads. Enter the quantity, the running power each (kW, kVA or motor hp) and its power factor.
  2. Tick "Motor" for loads that need starting and choose DOL, soft starter or VFD. The starting kVA multiple fills in automatically (6×, 3× and 1.5×). Replace it with the motor's locked-rotor data if you have it.
  3. Set the diversity factor (1 if everything can run at once), the design margin (20–25 % is common), and any derating % for altitude or temperature from the generator maker's data.
  4. Leave starting capability at 1.0 for a conservative result. Raise it only to match the maker's motor-starting curve at the voltage dip you can accept.
  5. Read the minimum rating and the next common size, then check which requirement governs.

How it works: the formula

Per row: P = qty × kW (hp → kW = hp × 0.7457 / η_motor) Q = P × tan(acos PF) Running: S = √((ΣP)² + (ΣQ)²) × DF S_run = max(S, ΣP × DF / PF_gen) × (1 + margin) / (1 − derate) Starting: S_start = multiple × S_motor (one unit) S_peak = |other running load| × DF + S_start S_st,req = S_peak / capability / (1 − derate) Required: S_req = max(S_run, S_st,req) → next common kVA

The set must cover two separate limits. When running, the engine must supply the kW and the alternator the kVA. A generator rated 100 kVA at PF 0.8 can only supply 80 kW, which is why both conditions are checked. When a motor starts, it briefly draws several times its running kVA at low power factor. The largest such step, added on top of everything else that is running, usually decides the size of sets serving motors.

Variables and units

SymbolMeaningUnit
P, Q, SReal, reactive and apparent powerkW, kVAr, kVA
DFDiversity factor: fraction of connected load running at once—
PF_genGenerator rated power factor (normally 0.8)—
multipleStarting kVA ÷ running kVA of a motor×
capabilityMomentary starting kVA the set can deliver ÷ rated kVA×
margin, derateDesign margin, and output reduction for site conditions%

Worked example

Loads: lighting 10 kW at PF 1.0; a 15 kW pump at PF 0.85 started direct on line (6×); a 7.5 kW fan at PF 0.8 on a soft starter (3×). Diversity 1, margin 20 %, no derating, starting capability 1.0.

ΣP = 10 + 15 + 7.5 = 32.5 kW ΣQ = 0 + 15 × 0.6197 + 7.5 × 0.75 = 9.296 + 5.625 = 14.921 kVAr S = √(32.5² + 14.921²) = 35.76 kVA S_run = max(35.76, 32.5 / 0.8 = 40.63) × 1.20 = 48.75 kVA Pump start: 6 × (15 / 0.85) = 6 × 17.647 = 105.88 kVA Other load: √(17.5² + 5.625²) = 18.38 kVA S_peak = 18.38 + 105.88 = 124.26 kVA → governs Next common size: 125 kVA

Motor starting sets the size here, not running load. With a soft starter on the pump (3×) the peak would fall to 18.38 + 52.94 = 71.3 kVA, and a set of about 75 kVA would cover both running and starting. That is the usual reason starting methods matter so much for generator projects.

Assumptions

  • Motors start one at a time, with the worst one last.
  • Starting kVA is added arithmetically to the running kVA of other loads. This overstates the true vector sum a little, which errs on the safe side.
  • The rating list is a common market list of kVA sizes, not any single manufacturer's range.

Limitations

  • Real generator selection uses the manufacturer's sizing software or published motor-starting curves. These relate starting kVA to transient voltage dip and frequency dip.
  • Voltage dip limits (for example 15–20 % for lighting or 30–35 % for some motor contactors) are not modelled directly. Starting capability is a simple stand-in for them.
  • Non-linear loads (UPS, VFDs, rectifiers) may need a larger alternator to limit voltage distortion.
  • Step loads with large block load acceptance, such as fire pumps, may need separate checks under ISO 8528-5 performance classes.
  • Prime, standby and continuous ratings have defined meanings (ISO 8528-1). Make sure the rating you compare against matches your duty.

References

Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.

  • ISO 8528-1, Reciprocating internal combustion engine driven alternating current generating sets — Part 1: Application, ratings and performance (rating definitions).
  • ISO 8528-5, Part 5: Generating sets — transient performance and load acceptance.
  • IEEE Std 446 ("Orange Book"), Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications.
  • NEMA MG 1, Motors and Generators — locked-rotor kVA code letters (also referenced in NEC 430.7(B)).

Frequently asked questions

Why is a DOL motor multiple 6×?

Induction motors started direct on line typically draw five to eight times full-load current at low power factor until they approach speed. 6× is a common planning value. The motor's code letter (NEMA MG 1) or the maker's locked-rotor current gives the real figure.

Does a VFD remove the starting problem?

Largely. A drive ramps the motor up and can limit current to about full-load current, so 1–1.5× is typical. Drives draw harmonic current, though, and that can need alternator oversizing. Check with the generator supplier.

What diversity factor should I use?

Use 1.0 unless you know some loads never run together. Diversity is project-specific. For standby sets serving life-safety loads, codes may require you to assume every connected load runs.

Why is there a margin as well as derating?

Margin covers future load growth and uncertainty in the load data. Derating covers the set producing less power at high altitude or high air temperature. They are separate effects, so the calculator applies both.

Can I trust the 'next common size'?

It is the next value in a common list of market kVA ratings. Actual ranges differ between manufacturers, and one maker's '125 kVA' may be another's '130 kVA'. Always confirm against a specific product's datasheet.

Related calculators

How these formulas were checked: methodology and testing.